deep-red solution to a liter beaker, dilute to about 400 c.c. with cold water, add 7 c.c. of 1 molar sodium acetate solution, then add 300 c.c. boiling water and heat the solution to boiling as quickly as possible, and continue the boiling for just three minutes. Filter the hot solution at once through a large fluted filter paper and wash the precipitate a few times with 0.1 molar sodium acetate solution. Dissolve the precipitate in as little hot 3 molar hydrochloric acid as possible, and repeat the basic acetate precipitation, washing the precipitate thoroughly with hot 0.1 molar sodium acetate solution. Add this filtrate to the first filtrate and concentrate the combined filtrates to about 250 c.c. If during this evaporation a small precipitate separates out of the solution, it must be removed by filtration. This precipitate is then dissolved in a small amount of 3 molar hydrochloric acid and another basic acetate separation is made in a solution the total volume of which is not over 50 c.c. Filter and add the filtrate to the main portion. Transfer the combined filtrates to a large Erlenmeyer flask, add about 15 c.c. of 2 molar sodium acetate solution, heat to 90°-95°, and then saturate with hydrogen sulphide under pressure.26 Digest until any precipitate coagulates and settles, filter while hot, and wash the precipitate with water saturated with hydrogen sulphide.

The next step is the precipitation of the manganese as manganese dioxide by means of bromine. This operation might seem unnecessary in the case of a spiegel since the latter contains no zinc, magnesium, or calcium to give insoluble phosphates, but there is such an accumulation of sodium salts at this juncture that a reprecipitation of the manganous ammonium phosphate would be necessary; hence it is a better plan to throw out the manganese at this point as the dioxide. Boil out the hydrogen sulphide from the filtrate, allow the latter to cool, and add 2-3 c.c. bromine (not bromine water); heat to expel the excess of bromine and filter. To the filtrate add a few drops more of bromine and a few drops of 2 molar sodium carbonate solution in order to make the filtrate slightly more alkaline so as to insure the complete precipitation of the manganese. If more manganese dioxide

26 The saturation with hydrogen sulphide may be omitted if it is known that nickel and cobalt are absent.

is obtained, filter and wash same, and add it to the main portion. Lissolve the manganese dioxide from the filter with hot 3 molar hydrochloric acid containing a little sulphurous acid.

The hydrochloric acid solution is next diluted to about 150 c.c. and a solution of di-ammonium hydrogen phosphate (NH4)2HPO4 is added in considerable excess, e.g., about five times the theoretical amount necessary to combine with the manganese.27 If a precipitate should form at this point, it is dissolved by the cautious addition of 3 molar hydrochloric acid. The solution is now heated to 90°-95° on the water-bath and 3 molar ammonium hydroxide added dropwise and with constant stirring until a flocculent precipitate begins to form; this precipitate is Mn3(PO4)2. The heating is continued until the precipitate becomes changed over into the crystalline MnNH PO4 H2O; then another drop of ammonium hydroxide is added and so on until further additions cause no appreciable change. Care must be taken, however, that in the end there shall be only a very slight excess of ammonium hydroxide. The precipitate of MnNHPO4 H2O is nearly white with a characteristic nacreous luster and is one of the easiest precipitates known to filter and wash. If an excess of ammonium hydroxide has been added, the precipitate may be brown in color, in which case add 3 molar hydrochloric acid until the precipitate dissolves and repeat the precipitation as described above. As soon as the precipitate has become crystalline in character, the solution is allowed to cool to room temperature and is then filtered through a Gooch crucible and washed with 0.1 molar ammonium nitrate solution until the washings give no test for chloride ion. The precipitate may be dried at 100°-105° and weighed as MnNH PO4 H2O, or it may be ignited at the full heat of the Meker burner and weighed as Mn2P207.

298. Examples.

1. In determining the percentage of manganese in an ore by Volhard's method, a standard solution of potassium permanganate (1 c.c. = 0.010033 g. Na2C2O4) was used as a source to supply a known weight of manganous ion, the manganous ion being then oxidized to acid manganite ion (manganese dioxide) by means of the same standard permanganate solution. The pro

27 1 c.c. 0.1 M (NH4)2HPO4 = 5.5 mg. Mn. The di-ammonium hydrogen phosphate solution may be readily prepared by neutralizing phosphoric acid with ammonium hydroxide until the solution just turns pink when phenolphthalein is used as the indicator.

cedure was as follows: known weights of sodium oxalate were dissolved in 50 c.c. of water and 2 c.c. of 9 molar sulphuric acid added; the solution was heated to 90° and titrated with the permanganate solution. The results were:

After titration each of the solutions was neutralized to methyl orange with 2 molar sodium carbonate, 25 c.c. 2 molar zinc sulphate solution were added and the solution heated to 90°-95°; the permanganate solution was then added until the end point was reached. The results were:

How many grams of manganese does 1 c.c. of the permanganate solution oxidize? What was the average deviation? How many grams of manganese should 1 c.c. of the permanganate solution oxidize theoretically? What is the constant error? Ans. 0.002498 g. Mn; 1.2 parts per 1000 0.002469 g. Mn; 11.6 parts per 1000

2. The standard permanganate solution of Example 1 was used in the determination of manganese in an ore by Volhard's method. 1.002 g. sample was taken, dissolved, etc., and the solution made up to 1000 c.c. Aliquot portions of 200 c.c. were taken, the p1 adjusted, 25 c.c. of 2 molar zinc sulphate solution added to each, and the portions heated to 90°-95° and titrated with the permanganate solution. The results were:

What was the percentage of manganese in the ore? deviation?

What was the average

Ans. 58.77%; 1.7 parts per 1000

3. In determining the percentage of manganese in a spiegel by Volhard's method, 1.000 g. of sample was taken, dissolved, etc., and the solution made up to 1000 c.c. Aliquot portions of 200 c.c. were taken, the p adjusted, 25 c.c. 2 M ZnSO4 added, etc., and titrated with a potassium permanganate solution, 1 c.c. of which would oxidize 0.001655 g. Mn++ to MnO2 by Volhard's method; 22.31 c.c. were required. What was the percentage of manganese? Ans. 18.46%

CHAPTER XIX

IODIMETRIC METHODS

STANDARDIZATION OF SODIUM THIOSULPHATE SOLUTION.

OF COPPER IODIMETRICALLY

DETERMINATION

299. General Remarks. The reaction 2 I- I finds frequent application in analysis because of the fact that the forward reaction can be effected quantitatively by many of the important oxidizing agents, while the reverse action can likewise be effected quantitatively by many of the important reducing agents, and because of the further fact that the disappearance or appearance of the merest concentrations of iodine in such reactions is accompanied by the simultaneous disappearance or appearance of a pronounced blue color if two or three c.c. of a one-percent starch solution are present at the time.

As illustrations of the use of the forward reaction 2 I-→ I may be mentioned the quantitative reduction of the following ions: permanganate, dichromate, hypochlorite, ferricyanide, arsenate, antimonate, and cupric ions and of hydrogen peroxide. In these cases the iodine which is liberated is titrated by means of standard sodium thiosulphate solution until only a small amount of the iodine remains unreduced, whereupon the starch solution is added and the titration completed.

As illustrations of the use of the reverse reaction 2 I-← I; may be mentioned the quantitative oxidation of the following ions: arsenite, stannous, sulphide, sulphite and thiosulphate. In these cases a standard solution of iodine is added to the reducing agent in the presence of the starch solution until the blue color is developed, or the reducing agent is added to a measured amount of standard iodine solution which is known to be in excess, and the excess of iodine titrated back with standard sodium thiosulphate solution, the starch solution not being added until the stoichiometrical point is approached.

300. Starch Indicator. The color which small concentrations of iodine give in the presence of small concentrations of starch solution was formerly thought to be due to the formation of an iodo-starch compound but is now believed to be due to the formation of a colloidal sol made up of the soluble portion of the starch (8-amylose), iodide ion, and iodine, and having the general composition (6-amylose)p(iodide ion)q (iodine)r (H2O)s in which the values of q and r are small relative to p. The important thing is that all four of these factors must be present for the development of the blue color; thus if iodine is present but no iodide ion, there will be no blue color.1

Now starch yields in addition to ẞ-amylose (granulose), which is the soluble portion of starch and apparently a pure carbohydrate, another product called a-amylose (amylopectin or starch cellulose), which is insoluble and seems to be a carbohydrate in combination with a fatty acid of the nature of oleic or linoleic acid.

This a-amylose gives a reddish color with iodine which is not discharged nearly as readily as the blue color of the B-amylose, and in this sense it is objectionable; fortunately, however, a-amylose constitutes only a small fraction of starch, say from 2% in potato starch to 15% in corn starch. Hence, by the simple choice of potato starch, the disturbing influence due to a-amylose is rendered negligible.

Starch, which has the empirical formula (C6H1005), occurs in the cells of many plants in the form of circular or elongated microscopic granules. These granules are insoluble in water and it is only when heated up to 50° with water that they burst and yield the insoluble a-amylose and the soluble ẞ-amylose. Both these products are quite rapidly hydrolyzed in aqueous solution into their degradation products amylodextrin, erythrodextrin, achroodextrin, etc. if the concentration of hydrogen ion is greater than 10-2. All these degradation products give with iodine red colors which are not discharged at the stoichiometrical point. Hence another reason why, in iodine titrations, the starch solution should not be added until the stoichiometrical point is just about to be reached; this is particularly apropos when the titration is being conducted in an acid solution as is often the case.

1 Lottermoser, Zeit. angew. Chem. 37, 84 (1924); Z. Elektroch. 27, 496 (1921).